(1) Field of the Invention
The present invention relates to pneumatic tires in which durability of bead portions is improved.
(2) Related Art Statement
As pneumatic tires in which durability of bead portions are improved, tires described in GB 1210935have been conventionally known. In each of those tires, a carcass ply has a main portion located axially inside beads and outer ply portions extending along the main portion and axially outside the beads, respectively. In the tire, recesses are formed in an outer surface of the tire and are spaced radially outwardly from a radially outer end of each of the outer ply portions by 10-50 mm and that the depth of the recesses is 1/10 times to 1/2 times as large as the thickness of those portions of the tire which are provided with the recesses. The recesses may circumferentially continue to one another or a number of recesses may be provided while being circumferentially spaced from one another.
The above-mentioned recess or recesses can improve durability of the bead without any problem in the case of the pneumatic tire described in GB 1210935 in which an inclined angle of a base portion of the bead is about 5.degree. relative to a rotary axis of the tire. However, if such a recess or recesses are formed, without paying any consideration thereto, in the case of a pneumatic tire in which an inclined angle of a base portion of a bead is about 15.degree. relative to a rotary axis of the tire and of which practical use conspicuously increases, the following problems occur. That is, since the pneumatic tire having the above inclined angle of about 5.degree. is fitted to a rim having a considerably high rim flange being about 0.038 times to about 0.055 times as high as the diameter of a base portion of the rim, great forces occur between the bead portion of the tire and the rim flange to assuredly join the tire and the rim. Consequently, it is unnecessary to particularly increase the thickness of the bead portion in order to prevent slipping between the tire and the rim. However, since a rim flange of a fitting rim is low (about 0.020 times to about 0.029 times as high as the diameter of the base portion of the rim) in the case of the pneumatic tire having the above inclined angle of about 15.degree., tire-rim joining forces occurring between the bead portion of the tire and the rim flange are small. Therefore, in order that slipping between the tire and the rim as well as failure of the bead portion resulting from the slipping may be prevented by generating sufficient tire-rim joining forces between the base portion of the bead of the tire and a bead seat portion of the rim, the thickness of the base portion of the bead is increased. Further, in order that the bead portion may not be damaged owing to concentration of deformation in a rotational direction of the tire during rotating under load through rapid changes in rigidity in circumferential direction of the bead portion, the thickness of the bead portion is gradually decreased from near the base portion of the bead to a location corresponding to the maximum width of the tire. Although every portion constituting the bead portion shares the function to improve durability of the bead portion, problems as mentioned below occur if recesses described in the prior art are formed, without any particular consideration, in the tire having the inclined angle of the base portion of the rim being about 15.degree. relative to the rotary axis of the tire. For example, if a recess is continuously formed in the circumferential direction, rigidity of the bead greatly decreases and deformation concentrates upon the tire during rolling under load in the rotating direction of the tire because the bead portion has the structure in which the thickness is inherently great and the rigidity in the circumferential direction is great. As a result, cracks (diagonal cracks) occur in the bottom of the recess in directions oblique to the circumferential direction of the tire, and these cracks progress into the interior of the tire to lower durability of the bead portion. Further, if a number of recesses are formed such that they are spaced from one another in the circumferential direction, the recesses cannot exhibit the effect of lowering the compression strain at the radially outer end of the outer ply portion depending upon the dimension of the recesses, with the result that durability of the bead portion cannot be improved, or the above diagonal cracks occur due to reduction in rigidity of the bead portion in the circumferential direction to lower the durability of the bead portion.